Virus infections progress from one host to another by methods already familiar in bacterial and protozoan diseases. Some are transmitted by direct contact through the skin and mucous membranes, others are spread by secretions from the upper respiratory tract, by the bites of insects and by coitus. There is, however, a general absence of virus diseases of the gastrointestinal tract of man.
The basic immunologic phenomena of virus diseases are also similar to those operative in other fields of biology. Some virus diseases—for example rabies and vaccinia—attack many species of hosts; others are highly specific for certain species.1 Climate, sex, age, nutrition and genetic factors have all been shown to enter into the picture of natural immunity to certain virus infections. In man recovery from virus infection is usually followed by enduring active immunity, which in many instances may be operative during the remainder of an individual's life. Exceptions